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<h2><a name="SECTION0020620000000000000000"></a><a name="opener-director-objects"></a>
<br>
18.6.2 OpenerDirector Objects 
</h2>

<p>
<tt class="class">OpenerDirector</tt> instances have the following methods:

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3970' xml:id='l2h-3970' class="method">add_handler</tt></b>(</nobr></td>
  <td><var>handler</var>)</td></tr></table></dt>
<dd>
<var>handler</var> should be an instance of <tt class="class">BaseHandler</tt>.  The
following methods are searched, and added to the possible chains (note
that HTTP errors are a special case).

<p>

<ul>
<li><tt class="method"><var>protocol</var>_open()</tt> --
    signal that the handler knows how to open <var>protocol</var> URLs.
</li>
<li><tt class="method">http_error_<var>type</var>()</tt> --
    signal that the handler knows how to handle HTTP errors with HTTP
    error code <var>type</var>.
</li>
<li><tt class="method"><var>protocol</var>_error()</tt> --
    signal that the handler knows how to handle errors from
    (non-<code>http</code>) <var>protocol</var>.
</li>
<li><tt class="method"><var>protocol</var>_request()</tt> --
    signal that the handler knows how to pre-process <var>protocol</var>
    requests.
</li>
<li><tt class="method"><var>protocol</var>_response()</tt> --
    signal that the handler knows how to post-process <var>protocol</var>
    responses.
</li>
</ul>
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3971' xml:id='l2h-3971' class="method">open</tt></b>(</nobr></td>
  <td><var>url</var><big>[</big><var>, data</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Open the given <var>url</var> (which can be a request object or a string),
optionally passing the given <var>data</var>.
Arguments, return values and exceptions raised are the same as those
of <tt class="function">urlopen()</tt> (which simply calls the <tt class="method">open()</tt> method
on the currently installed global <tt class="class">OpenerDirector</tt>).
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3972' xml:id='l2h-3972' class="method">error</tt></b>(</nobr></td>
  <td><var>proto</var><big>[</big><var>,
                                          arg</var><big>[</big><var>, ...</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Handle an error of the given protocol.  This will call the registered
error handlers for the given protocol with the given arguments (which
are protocol specific).  The HTTP protocol is a special case which
uses the HTTP response code to determine the specific error handler;
refer to the <tt class="method">http_error_*()</tt> methods of the handler classes.

<p>
Return values and exceptions raised are the same as those
of <tt class="function">urlopen()</tt>.
</dl>

<p>
OpenerDirector objects open URLs in three stages:

<p>
The order in which these methods are called within each stage is
determined by sorting the handler instances.

<p>

<ol>
<li>Every handler with a method named like
    <tt class="method"><var>protocol</var>_request()</tt> has that method called to
    pre-process the request.

<p>
</li>
<li>Handlers with a method named like
    <tt class="method"><var>protocol</var>_open()</tt> are called to handle the request.
    This stage ends when a handler either returns a
    non-<tt class="constant">None</tt> value (ie. a response), or raises an exception
    (usually <tt class="exception">URLError</tt>).  Exceptions are allowed to propagate.

<p>
In fact, the above algorithm is first tried for methods named
    <tt class="method">default_open</tt>.  If all such methods return
    <tt class="constant">None</tt>, the algorithm is repeated for methods named like
    <tt class="method"><var>protocol</var>_open()</tt>.  If all such methods return
    <tt class="constant">None</tt>, the algorithm is repeated for methods named
    <tt class="method">unknown_open()</tt>.

<p>
Note that the implementation of these methods may involve calls of
    the parent <tt class="class">OpenerDirector</tt> instance's <tt class="method">.open()</tt> and
    <tt class="method">.error()</tt> methods.

<p>
</li>
<li>Every handler with a method named like
    <tt class="method"><var>protocol</var>_response()</tt> has that method called to
    post-process the response.

<p>
</li>
</ol>

<p>

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